Monday, May 23, 2011

A REPL for the RHQ Plugin Container

RHQ plugins run inside of a plugin container that provides different services and manages the life cycles of  plugins. The plugin container in turn runs inside of the RHQ agent. If you are not familiar with the agent, it is deployed to each machine you want RHQ to manage. You can read more about it here and here. While the plugin container runs inside of the agent, it is not coupled to the agent. In fact is it used quite a bit outside of the agent. It is used in Embedded Jopr which was intended to be a replacement for the JMX web console in JBoss AS. The plugin container is also used a lot during development in automated tests.

My teammate, Heiko Rupp, has developed a cool wrapper application for the plugin container. It defines a handful of commands for working with the plugin container interactively. What is nice about this is that it can really speed up plugin development. Heiko has written several articles about the standalone container including Working on a standalone PluginContainer wrapper and I love the Standalone container in Jopr (updated). After reading some of his posts I got to thinking that a REPL for the plugin container would be really nice but not just any REPL though. I was thinking specifically about Clojure's REPL.

I have spent some time exploring the different ways Clojure could be effectively integrated with RHQ. There is little doubt in my mind that this is one of them. I recently started working on some Clojure functions to make working with the plugin container easier. I am utilizing Clojure's immutable and persistent data structures as well as some of the other great language features such as first class functions and multimethods. I am trying to make these functions easy enough to use so that someone who might not be a very experienced Clojure programmer might still find them useful during plugin development and testing.

Getting the Code
The project is available on github at It is built with leiningen so you want to get it installed. I typically run a swank server and connect from Emacs, but you can also start a REPL session directly if you are not an Emacs user. The project pulls in the necessary dependencies so that you can work with plugin container-related classes as you will see in the following sections.

Running the Code
These steps assume that you already have leiningen installed. First, clone the project:

git clone

Next, download project dependencies with:

lein deps

Some plugins reply  on native code provided by the Sigar library which you should find at clj-rhq/lib/ Create a directory in lib named native and unzip there. The project is configured to look for native libraries in lib/native.

Finally, if you are using Emacs run lein swank to start a swank server; otherwise, run lein repl to start a REPL session on the command line.

Starting/Stopping the Plugin Container

The first thing I do is call require to load the rhq.plugin-container namespace. Then I call the start function. The plugin container emits a line of output, and then the function returns nil. Next I verify that the plugin has started up by calling running?.  Then I call the stop function to shutdown the plugin container and finally call running? again to verify that the plugin container has indeed shutdown.

Executing Discovery Scans
So far we have looked at starting and stopping the PC. One of the nice things about working interactively in the REPL is that you are not limited to a pre-defined set of functions. If rhq.plugin-container did not offer any functions for executing a discovery scan, you could write something like the following:

The pc function simply returns the plugin container singleton which gives us access to the InventoryManager. We call InventoryManager's executeServerScanImmediately method and store the InventoryReport object that it returns in a variable named inventory-report. Alternatively you can use the discover function.

On the first call to discover we pass the keyword :SERVER as an argument. This results in a server scan being run. On the second call, we pass :SERVICE which results in a service scan being run. If you invoke discover with no arguments, a server scan is executed followed by a service scan. The two inventory reports from those scans are returned in a vector. The use of the count function to see how many resources were discovered is a good example that demonstrates how you can easily use functions defined outside of the rhq.plugin-container namespace to provide additional capabilities and functionality.

Searching the Local Inventory
Once you have the plugin container running and are able to execute discovery scans, you need a way query the inventory for resources with which you want to work. The inventory function does just that. It can be invoked in one of two ways. In its simpler form which takes no arguments, it returns the platform Resource object. In its more complex form, it takes a map of pre-defined filters and returns a lazy sequence of those resources that match the filters.

inventory is invoked on line 1 without any arguments, and then a string version of it is returned with a call to str. The type is Mac OS X indicating that the object is in fact the platform resource. On line 5 we invoke inventory with a single filter to include resources that are available.  That call shows that there are 62 resources in inventory that are up.  On line 7 we query for resources that are a service and see that there are 60 in inventory. On line 9 we specify multiple filters that will return down services. When multiple filters are specified, a resource must match each one in order to be included in the results. On line 10 we query for webapps from the JBossAS plugin. On line 13 we specify a custom filter in the form of an anonymous function with the :fn key. This filter finds resources that define at least two metrics.

We have looked at a number of functions to make working with the plugin container from the REPL a bit easier. Each function should also include a useful docstring as in the following example,

We have only scratched the surface with the functions in the rhq.plugin-container namesapce. In some future posts we will explore invoking resource operations, updating resource configurations, and deploying resources like EARs and WARs.

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